The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
(1) Field of the Invention
This invention generally relates to sonar systems and more specifically to a method and apparatus for determining whether recently acquired sonar contact information represents the acquisition of a new contact or the reacquisition of a previous contact that was lost.
(2) Description of the Prior Art
The use of sonar systems for tracking targets or contacts is well known, as are a wide variety of factors that effect the information derived from the sonar signals. Many of these factors cannot be controlled and include seawater temperature, salinity and other variables pertaining to seawater.
With early sonar systems, operators provided information about vessel range, bearing and identification using their experience and a priori knowledge to compensate for such factors. Over the years, however, a number of improvements have been made to automate certain analyses of sonar signals thereby to increase the accuracy and quality of an operator""s interpretation.
For example, U.S. Pat. No. 5,566,134 (1996) to Dufault discloses an algorithm that detects, enhances, tracks and displays both stable and unstable passive sonar signals while organizing them into dynamically related groups. Processing includes power spectrum analysis on a segmented time-wave-form from a hydrophone, processing of the spectra for detection and enhancement of the signal by various methods and formatting and displaying the detected and enhanced signals. More specifically, Automatic Band Tracking algorithms (ABT) detect persistent signals having related features by means of correlation and enhance the signals by mathematical integration (or smoothing). Signals belonging to several unrelated sets may be processed by successive ABTs with each ABT simultaneously displaying its processed portion of the total signal-content while removing precisely that signal from the spectrum to which the succeeding ABTs are assigned.
U.S. Pat. No. 5,914,912 (1999) to Yang discloses a sonar array post processor that employs adaptive processing, called matched beam processing, in the beam domain which takes analog or digital conventional beam-forming outputs from a sonar array, multiplies them by a set of adaptive weighting coefficients and produces a new set of beams. A new output beam of highest intensity yields the ideal maximum signal gain and correct target bearing. Continuous target tracking provided by the sonar array post processor in the endfire direction of a horizontal line array minimizes towing ship maneuvers and for a bottom mounted surveillance system requires a smaller number of arrays.
In U.S. Pat. No. 5,949,739 (1999) to Reese an improved bearing estimation sonar system provides high resolution imaging of extended multi-highlight targets at long ranges. The complex outputs of a pair of offset-phase centered beams are first processed by segmented replica correlators to achieve pulse compression, followed by cross-correlation to provide high fidelity bearing estimates for target imaging. The technique exploits space-time coherence properties of the signal received from the target to extract high resolution target information. Data editing schemes enhance discernability of submarine-like targets which is essential for target classification.
U.S. Pat. No. 5,991,238 (1999) to Barr discloses a method for attenuating water column reverberations in a dual sensor seismic signal whereby a pressure signal and a velocity signal are transformed from a time domain to a frequency domain and for generating a transformed pressure signal and a transformed velocity signal, respectively. Values for weighting factors, Kp and Kv, are selected and multiplied times the transformed pressure signal and the transformed velocity signal, respectively, generating a weighted pressure signal and a weighted velocity signal, respectively. The weighted pressure signal and the weighted velocity signal combine to generate a summed signal. Values for ocean bottom reflectivity R and Z, the frequency domain delay operator for two-way travel time in the water layer, are determined. A weighted inverse Backus filter is calculated and multiplied times the summed signal to generate a filtered signal that is transformed from the frequency domain to the time domain.
In U.S. Pat. No. 6,041,019 (2000) to Jackson et al. a method of data fusion determines a best-estimate solution to a moving contact using a plurality of trackers that provide bearing-to-contact data and range-to-contact data. An averaging function applied to the bearing-to-contact data determines weighted-average bearing data associated with each tracker. The same averaging function applied to the range-to-contact data determines weighted-average range data. A computed solution to the moving contact is generated using the weighted-average range data and the weighted average bearing data from the one tracker having the lowest standard deviation. A root mean square (RMS) error in terms of bearing is then determined for the computed solution and each of a plurality of independently generated solutions using bearing-to-contact data from the one tracker having the lowest standard deviation. One of the computed solution or independently generated solutions that produces the lowest RMS error constitutes the best-estimate solution.
U.S. Pat. No. 6,078,281 (2000) to Milkovich et al. discloses Fast Fourier Transformation (FFT) processing for measuring FFT output phase standard deviation over a number of consecutive FFT runs. The system corrects output phase in an organized fashion for all potential signal filter offset positions while measuring changes in phase standard deviation and selects the filter offset where the minimum standard deviation occurs. Pseudo coherent integration enhances and locates the mean phase shift within the number of FFTs integrated and corrects all FFT runs by this mean shift value. The integration multiplies the magnitude of each FFT filter output by the cosine and sums all FFT""s in the integration period for the respective filter. Detections are then declared based on outputs from a combination of a single filter detector, a sum coherent and traditional detector, a filtered coherent detector and a filtered traditional detector, thereby utilizing various blended techniques across the filter bandwidth.
In U.S. Pat. 6,096,085 a sonar simulator includes a parameter definition code for defining a plurality of parameters of a sonar, target and sonar environment and a signal-to-noise ratio (SNR) computation code for computing a SNR of the sonar as a function of range to target based upon the parameters defined by the parameter definition code including ambient noise, the volume scattering strength of the sonar environment, a sound velocity profile of the sonar, beam patterns of both projector and receiver of the sonar, the sonar type and range resolution, the number of eigenrays striking the surface and bottom of the sonar environment and the target, ray trajectories to the target and surface and bottom scattering strength as a function of angle. A target strength model computes scattering for a selected complex target contained in a stored set of complex target selections thereby to generate a target strength model for the selected complex target.
The foregoing references disclose methods and apparatus for improving sonar signal analysis by the enhancement of a particular function. Primarily each reference suggests improving the data obtained from existing signals. However, various environmental factors, ship maneuvers and other phenomena can cause a signal from a particular target under analysis to be lost. In such an operating condition there is a question as to whether the acquisition of a contact represents the acquisition of a new contact or the reacquisition of a contact that had been lost some time earlier, commonly called a xe2x80x9cfaded contactxe2x80x9d. Notwithstanding the foregoing improvements, the process for classifying such acquired contacts as a new contact or a reacquired faded contact continues to be an essentially manual operation. That is, the classification remains primarily a function of operator experience. What is needed is a method and apparatus for facilitating and automating this classification process to enhance the quality of information derived from sonar signals.
Therefore it is an objective of this invention to provide a method and apparatus for providing enhanced solutions to the classification of recently acquired contacts in sonar signals.
Yet another objective of this invention is to provide a method and apparatus for automating the classification of recently acquired contacts in a sonar system.
Yet still another objective of this invention is to provide a method and apparatus for enhancing the classification of a recently acquired contact in a sonar system that minimizes the requirement for manual intervention and analysis.
In accordance with an aspect of this invention, the classification of a newly acquired contact from a sonar system as either a new contact or a reacquired faded contract includes the establishment of a record of faded contacts. An identification of associations between the recently acquired contact and each recorded faded contact is made. An analysis of the identified associations enables the classification of the newly acquired contact as being a new contact, a reacquired faded contact or a contact of indeterminate origin.